BMI Calculator
Use this BMI calculator to work out your body mass index from your height and weight in seconds. Switch between metric and imperial units, see your category on a standard WHO chart, and get a healthy weight range for your height — all calculated instantly in your browser.
Your details
Choose your unit system, enter your height and weight, then calculate.
Your result
Reference: adult BMI category bands
These bands follow the WHO-style adult cutoffs used throughout this page. Fill in the form and calculate to see exactly where your number falls.
Health disclaimer: this BMI calculator provides an estimate only and is not medical advice or a diagnosis. BMI does not account for muscle mass, bone density, age, sex, or where your body stores fat — two people with the same BMI can have very different body compositions and health risks. Always consult a qualified healthcare professional before making decisions about your weight or health.
What BMI is — and the formula behind it
Body mass index (BMI) is a simple ratio of your weight to the square of your height. It compresses two everyday measurements into a single number that public-health bodies — including the World Health Organization — use to sort large populations into broad weight categories. It's quick to calculate, requires no special equipment, and gives a useful starting point for thinking about weight-related health risk.
The maths is the same one our calculator runs behind the scenes:
- Metric: BMI = weight in kilograms ÷ (height in metres)². For example, 70 kg at 1.75 m gives 70 ÷ (1.75 × 1.75) ≈ 22.9.
- Imperial: BMI = 703 × weight in pounds ÷ (height in inches)². For example, 154 lb at 69 inches gives 703 × 154 ÷ (69 × 69) ≈ 22.7.
Because the formula divides by height squared, taller and shorter people are compared on a level playing field — which is exactly why the "healthy weight" in kilograms or pounds looks so different from one person to the next, even when their BMI is identical.
BMI category chart (WHO bands)
Once you have a BMI number, it's placed into one of four standard adult bands. This is the same chart referenced in the gauge above:
| Category | BMI range | What it generally means |
|---|---|---|
| Underweight | Below 18.5 | May indicate insufficient energy intake or an underlying condition — worth discussing with a clinician. |
| Healthy weight | 18.5 – 24.9 | Associated with the lowest average health risk in most adult population studies. |
| Overweight | 25.0 – 29.9 | A signal worth monitoring — often used as a prompt to review activity and diet, not a verdict. |
| Obese | 30.0 and above | Further split into class 1 (30–34.9), class 2 (35–39.9), and class 3 (40+); usually warrants professional guidance. |
Note: some health bodies use a lower overweight threshold (around 23) for South and East Asian populations, reflecting research that shows higher health risk at lower BMI values for these groups. If that applies to you, treat the standard bands above as a general guide rather than a precise cutoff.
Limitations of BMI
BMI is popular because it's simple — but that simplicity comes at a cost. The formula only ever sees two numbers: your height and your weight. It cannot tell whether that weight comes from muscle, fat, bone, or fluid, and it says nothing about where on your body that weight sits. Keep these limits in mind when you read your result:
- It doesn't separate muscle from fat. Athletes and very muscular people often score "overweight" or "obese" despite low body-fat levels, while some people in the "healthy" band carry excess fat with little muscle.
- It ignores age and sex. Body composition naturally shifts as people age, and men and women carry muscle and fat differently — none of which the formula adjusts for.
- It's not built for everyone. Standard adult thresholds aren't appropriate for children and teenagers (who need growth-chart-based percentiles), pregnant people, or some ethnic groups whose health risk profiles shift at different BMI values.
- It says nothing about fitness or habits. Two people with an identical BMI can have completely different fitness levels, diets, and chronic-disease risk. BMI is a screening flag, not a complete health picture.
How to read your result
When you calculate, you'll see three things: your BMI number, the category it falls into, and a healthy weight range for the height you entered. Treat the number as a conversation-starter rather than a final answer. If it lands close to a band edge — say 24.7 or 25.3 — you're not meaningfully "healthier" or "less healthy" than someone a tenth of a point on the other side; the bands are descriptive groupings, not precise thresholds.
The healthy weight range is simply the weight your current height would need to land inside the 18.5–24.9 band — it's the same formula run in reverse. It's a useful goalpost for context, but it's not a personal target: your ideal weight depends on your frame, muscle mass, health history, and goals, all of which a doctor or registered dietitian can help you weigh up properly.
Related health metrics worth checking
BMI is a useful first screen, but it works best alongside other numbers. Your daily energy needs (calories), body composition (fat percentage), and a frame-adjusted target weight all paint a fuller picture than any single measurement on its own — which is exactly why we built calculators for each of them.
Frequently asked questions
- What is a BMI calculator?
- A BMI calculator estimates your body mass index from your height and weight. It returns a single number and a category band (underweight, healthy, overweight, obese) used as a quick population-level screening tool. It does not measure body fat directly.
- How do I calculate BMI?
- In metric, divide your weight in kilograms by your height in metres squared (kg/m²). In imperial, multiply your weight in pounds by 703 and divide by your height in inches squared. Our calculator does both automatically when you switch units.
- What is a healthy BMI range?
- The World Health Organization defines a healthy BMI as 18.5 to 24.9. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is classed as obese. These bands apply to most adults aged 18 and over.
- Is BMI accurate for everyone?
- No. BMI does not separate muscle from fat, so very muscular athletes can read as overweight while some people in the healthy range carry excess fat. It is also not valid for children, pregnant people, or some ethnic groups without adjusted thresholds.
- What is a good BMI for my height?
- A good BMI for any height falls in the 18.5 to 24.9 band. Because BMI already accounts for height, the healthy *weight* range changes with height. Our calculator shows the exact healthy weight range in kg and lb for the height you enter.
- Does BMI differ for men and women?
- The BMI formula and the standard category thresholds are the same for adult men and women. Body composition differs by sex, which is one reason BMI is a screening tool rather than a diagnosis. For fat percentage, use our body fat calculator.
- What BMI is considered obese?
- A BMI of 30.0 or higher is classed as obese. This is further split into class 1 (30–34.9), class 2 (35–39.9), and class 3 (40+). A higher class is associated with greater health risk and usually warrants professional advice.
- Is this BMI result medical advice?
- No. This BMI calculator gives a screening estimate only and is not medical advice or a diagnosis. Discuss your weight and health with a qualified doctor or dietitian, especially before making major changes.